When applying a steering angle to steered wheels (generally, front wheels except for a special vehicle such as a forklift), a power steering device has been widely used as a device for reducing a force necessary for a driver to operate a steering wheel. As such power steering device, an electric power steering device configured to use an electric motor as an auxiliary power source has also been recently used. Various structures have been known for the electric power steering device. In any structure, auxiliary power of the electric motor is applied via a reducer to a rotary shaft configured to rotate in accordance with an operation of the steering wheel and to apply the steering angle to the steered wheels as it rotates. As the reducer, a worm reducer is generally used. According to the electric power steering device using the worm reducer, a worm configured to rotate by the electric motor and a worm wheel configured to rotate together with the rotary shaft are meshed with each other such that the auxiliary power of the electric motor can be transmitted to the rotary shaft. In the worm reducer, an uncomfortable abnormal noise called as gear striking noise may be generated when changing a rotational direction of the rotary shaft due to a backlash existing at the meshed portion between the worm and the worm wheel.
As a structure capable of suppressing the gear striking noise, it has been considered to elastically press the worm toward the worm wheel by an elastic member such as a spring. FIGS. 9 and 10 show an example of the electric power steering device disclosed in Patent Document 1. A front end portion of a steering shaft 2 configured to rotate in a predetermined direction by a steering wheel 1 is rotatably supported within a housing 3, and a worm wheel 4 is fixed to this part. Worm teeth 5 configured to mesh with the worm wheel 4 are provided at an axially intermediate portion of a worm shaft 6, and both axial end portions of a worm 8 configured to rotate by an electric motor 7 are rotatably supported in the housing 3 by a pair of rolling bearings 9a, 9b such as deep groove ball bearings or the like. A pressing piece 10 is externally fitted to a tip portion of the worm shaft 6, which protrudes more than the rolling bearing 9a, and an elastic member such as a coil spring 11 is provided between the pressing piece 10 and the housing 3. By the coil spring 11, the worm teeth 5 provided on the worm shaft 6 are pressed toward the worm wheel 4 via the pressing piece 10. By this configuration, the backlash between the worm teeth 5 and the worm wheel 4 is suppressed to reduce the generation of the gear striking noise.
According to the conventional structure, it is possible to suppress the generation of the gear striking noise at the meshing part between the worm teeth 5 and the worm wheel 4. However, it is not possible to suppress an abnormal noise, which is to be generated at a coupling portion between a tip portion of an output shaft 12 of the electric motor 7 and a base end portion of the worm shaft 6. This is described as follows. In the structure of FIG. 10, in order to couple the tip portion of the output shaft 12 of the electric motor 7 and the base end portion of the worm shaft 6 such that the torque can be transmitted, the base end portion of the worm shaft 6 is formed with a spline hole 13 opening toward a base end surface of the worm shaft 6.
The tip portion of the output shaft 12 is formed with a spline shaft portion 14. The spline shaft portion 14 and the spline hole 13 are spline-engaged, so that the output shaft 12 and the worm shaft 6 are coupled such that the torque can be transmitted.
When the spline shaft part 14 and the spline hole 13 are spline-engaged without a circumferential gap (without the backlash), the abnormal noise does not occur at the coupling portion (the spline engagement part) between the tip portion of the output shaft 12 and the base end portion of the worm shaft 6. However, in the actual situation, the backlash exists at the spline engagement part. Particularly, when the backlash between the worm teeth 5 and the worm wheel 4 is suppressed by the structure as shown in FIG. 10, it is not possible to completely remove the backlash of the spline engagement part because it is necessary to allow swingable displacement of the worm shaft 6.
Patent Document 2 discloses a structure where the output shaft of the electric motor and the worm shaft are coupled via a metallic cylindrical power transmission member such that the worm shaft can be smoothly swingably displaced. In the structure of Patent Document 2, since the worm shaft is swingably displaced, the backlashes exists at the spline engagement parts between the spline shaft portions (male splines) provided at both end portions of the power transmission member and the spline holes (female splines) provided at respective end portions of the worm shaft and the output shaft of the electric motor, respectively. Therefore, the abnormal noise may be generated when changing a rotational direction of the rotary shaft.